Rated capacity: the apparent power consumed when the rated secondary current passes through the secondary rated load. The rated capacity can be expressed by the apparent power V.A, or it can be expressed by the secondary rated load impedance Ω. Primary rated current: the electrical load current allowed to pass through the primary winding of the current transformer. The primary rated current of the current transformer used in the power system is 5 ~ 25000A, and the precision current transformer used in the test equipment is 0.1 ~ 50000A. The current transformer can be operated for a long time under a rated current. When the load current exceeds the rated current value, it is called overload. If the current transformer is overloaded for a long time, it will burn out the winding or reduce the service life.